Import ssl on demand (#3401)

Author: tomchristie

httpx/_config.py

@@ -1,12 +1,14 @@
 from __future__ import annotations
 
-import ssl
 import typing
 
 from ._models import Headers
 from ._types import HeaderTypes, TimeoutTypes
 from ._urls import URL
 
+if typing.TYPE_CHECKING:
+    import ssl  # pragma: no cover
+
 __all__ = ["Limits", "Proxy", "Timeout", "create_ssl_context"]